The present invention relates to testing substrates and in particular to measuring the surface profile of a substrate and the stress induced on a substrate due to processing.
Flat substrates, such as semiconductor wafers, are stressed during certain processing steps, e.g., depositing or etching thin films. Stress in deposited layers can warp the substrate, which can adversely affect subsequent process steps, device performance, reliability and line-width control. Thus, it is desirable to measure the surface profile of a substrate as well as measure the stress on a substrate that is associated with a processing step.
There are many measurement tools available for measurement of a surface profile and analysis of the stress associated with certain processing steps on substrates. Most of the available tools for the semiconductor industry use a laser displacement sensor to measure the surface profile and to monitor the change in radius of curvature of the wafer before and after the processing step. A laser displacement sensor relies on the reflectance of a laser beam from the surface of the sample to quantify the change in angle of incidence of the beam with respect to the wafer. This information can be transformed into an average radius of curvature for an entire diameter of a wafer or an average radius of curvature for a fraction of a diameter allowing the calculation of stress as a function of position along the diameter. This measurement can be repeated at a number of locations within a diameter and at a multitude of diameters to create a map of the stress over an entire wafer.
Laser displacement sensors typically require large optical components. Consequently, these systems are expensive and are generally stand-alone measurement and inspection tools.
It would be advantageous to measure surface profiles and to measure stress using an existing semiconductor metrology tool without adding additional hardware. This could be beneficial for an integrated metrology module (a measurement tool integrated into a process tool to perform a specific metrology function) to save space, reduce cost and increase functionality.
The focusing capabilities in a metrology tool may be used to determine the surface profile of a substrate and the stress on the substrate that is associated with a processing step, in accordance with an embodiment of the present invention. The surface profile is determined by using the amount of adjustment necessary to place the substrate in focus in the metrology tool at three or more locations. Because the focal distance of a metrology tool is a known fixed distance, the amount of adjustment necessary to place a substrate in focus can be used to determine the surface height at that location. Accordingly, a surface profile can be produced based on a plurality of locations on the sample. A curvature parameter, such as a radius of curvature can then be calculated based on the surface profile. The curvature parameters can be calculated for the substrate before and after a processing step. The stress associated with the processing step can then be determined from these two curvature parameters. Thus, no additional hardware is necessary for a metrology tool to perform a surface profile measurement and stress measurement in accordance with the present invention.
In one embodiment of the present invention, a method includes adjusting the focus of a metrology tool to focus on the surface of a substrate at least at three measurement points. The height of the surface of the substrate is determined for each measurement point based on the amount of adjustment necessary to focus the metrology tool on the surface. The radius of curvature can then be calculated for the measurement points. In addition, the method may include calculating a pre-processed radius of curvature, processing the substrate and calculating a post-processed radius of curvature. The pre-processed and post-processed radii of curvature can be compared to determine the difference and the stress associated with the processing can then be calculated based on the difference in the radius of curvature.
In another embodiment of the present invention, a method includes measuring a first surface profile of a sample based on the adjustment necessary to place the surface in focus in a metrology tool, where the surface profile is measured at least at three locations before a process is executed on the sample. A second surface profile is measured after the process is executed on the sample. A first curvature parameter is calculated from the first surface profile, and a second curvature parameter is calculated from the second surface profile. The stress on the sample associated with the execution of the process can then be calculated using the first and second curvature parameters.
In yet another embodiment of the present invention, an apparatus includes a stage for holding a substrate, where the stage is movable to place the substrate at desired locations. The apparatus also includes an objective lens positioned generally over the stage. The objective lens has a known focal distance and the distance between the stage and objective lens is variable to place the surface of the substrate at the focal distance of the objective lens. The apparatus also includes a computer system coupled to the stage and the objective lens to receive a signal indicating the location of the substrate and a signal indicating the distance varied to place the surface at the focal distance, the computer system having a computer-usable medium having computer-readable program code embodied therein for measuring a surface profile of the sample based on the distance varied to place the surface at the focal distance at each location of the substrate; and calculating a curvature parameter of the substrate from the surface profile. The computer-readable program code may also include calculating a first curvature parameter for the surface profile before the substrate is processed and calculating a second curvature parameter for the surface profile after the substrate is processed. The code compares the first curvature parameter and the second curvature parameter to determine a differential curvature parameter; and calculates the stress associated with the processing based on the differential curvature parameter.